Figure 4

Neither cleavage furrow initiation nor progression includes passage through the sperm microinjection point (MIP). (A1–A5) A Baboon zygote with radially aligned apposed pronuclei oriented obliquely to the A → V zygotic axis. A detached 3-micron bead within the perivitelline space marks the MIP (A1, black arrow). Before mitosis, the expanding cytoplasm re-engages the bead (A2, black arrow). An asymmetric cleavage furrow forms near the previously intact apposed pronuclei (A3, large arrowhead), progressing meridionally through the polar bodies (Pbs), but not the MIP (A3, black arrow). The attached bead rotates by surface membrane flow toward the furrow (A3–A4, black arrows). Post-cytokinesis, the embryo shows unequal cleavage, with polar bodies in the division plane and the bead near the furrow (A5, black arrow). (B1–B10). A Baboon zygote showing pronuclear events (B1–B5) and movement of two attached marking beads during cytokinesis (B6–B10). A 5-micron bead marks the MIP (B6, black arrow); the 3-micron bead marks a distal cortex site (B7, black arrowhead). Radially aligned apposed pronuclei are oblique to the A → V axis (B1) with a bead-marked MIP (B6: black arrow). The apposed pronuclei translocate cortically, aligning with the A → V axis pre-mitosis (B2). Slight membrane flow exposes both attached beads (B7, black arrow, arrowhead). Furrow initiation occurs near the formerly intact MPn (B3, white arrowhead), proceeding meridionally toward the polar bodies (Pbs), but not through the beads (B8, black arrow, arrowhead). Beads show independent movement; the 3-micron bead initially toward the polar bodies (B8, white curved arrow) before redirecting toward the furrow, while the 5-micron MIP bead moves vertically toward the mid-furrow (B9, small white arrows). Cytokinesis is unequal, with polar bodies (Pbs) between blastomeres and both beads near the furrow (B10, black arrow, arrowhead). (C1–C2). The A1–A5 Baboon zygote after in vitro development. Second division is equatorial in both daughter cells, with the MIP bead near the polar body between blastomeres (C1; black arrow). (D) The B1–B10 baboon blastocyst on Day 8 post-in vitro culture. All panels are HMC images. Time (hours:minutes) post-ICSI. Vac: cytoplasmic vacuole(s). White arrow: zona pellucida reference bead; White ^*zona pellucida surface debris. Bars = 20 μm.